Call-for-service and continuity sensor circuit

A call-for-service and continuity sensor circuit for use in a trunk circuit of a telephone switching system including at least first and second switching centers connected via said trunk circuit in ground start configuration, said trunk circuit having first and second leads connecting said switching centers, said call-for-service and continuity sensor circuit comprising:

• circuit input means including signal conditioning means connected to said first and said second leads of said trunk circuit;
• circuit sensing means including comparator means connected via included inputs to said signal conditioning means;
• said input means cooperating with said signal conditioning means to adapt signals sensed on said trunk circuit leads for use by said comparator means while protecting said trunk circuit from excessive loading and protecting said comparator means from excessive induced potentials;
• and
• circuit output means connected to an output of said comparator means and having an output connected to said second switching center, said circuit output means operated to convert signals from said comparator means to signals suitable for use by said second switching center and to couple said suitable signals to said second switching center via an output terminal;
• whereby, in response to the application of negative potential to said second lead and an open circuit to said first lead at said first switching center, said sensor is operated to produce a first output signal to said second switching center, and in response to the application of negative potential to said second lead and earth ground to said first lead at said first switching center, said sensor is further operated to produce a second output signal, and in response to the application of an open circuit to both of said leads at said first switching center, said sensor is still further operated to produce said second signal.